Various mobile or portable electronic devices may have reduced power consumption by operating some of the systems within these devices at low voltages (e.g., 3.0 volts, 1.5 volts, etc.). Such electronic devices often use direct current to direct current converters (“dc to dc converters” or “dc-dc converters”) to “step down” voltages available from their power supplies to the lower voltages used by these systems.
Generally, dc-dc converters may be operated in two modes. The so called pulse width modulation (PWM) mode allows for high load current, but tends to be less efficient at low load currents. The so called pulse frequency modulation (PFM) mode can generally provide only a small load current, but is more efficient at low load currents than the PWM mode. Some dc-dc converters are capable of operating in both modes of operation, and may switch from one mode of operation to the other mode of operation when load current demands change. For example, a dc-dc converter may operate in PWM mode when higher load currents are used and switch to the more efficient PFM mode when lower load currents are used.
Different techniques are used by dc-dc converters to determine when to switch from one operating mode to another. One way some dc-dc converters control mode changes is by monitoring the amplitude of the current through the coil (inductor) of the dc-dc converter, and switching modes in the dc-dc converter when the current reaches a given threshold. The assumption in this approach is that a measurement of the amplitude of the coil current at a midpoint of the rising current waveform represents the average coil current, and that the average coil current is a reasonable indicator of the load current. This approach may be sufficient, so long as current is flowing through the coil (referred to as “continuous conduction mode”). However, to avoid a negative current flowing in the coil (which would discharge the output capacitor, negatively affecting the efficiency of the dc-dc converter), some dc-dc converters clamp the coil current at zero, and prevent the current from becoming negative (referred to as “discontinuous conduction mode”). In discontinuous mode, a measurement of the amplitude of the coil current at a midpoint of the rising current waveform no longer represents the average coil current (since a portion of the cycle is clamped near zero), and therefore does not represent the load current.
In addition, to reduce a printed circuit board (PCB) footprint and/or to reduce a bill of materials in some designs, there is a trend for some designers to decrease the size of the inductors used in dc-dc converters. A smaller coil (with less inductance) leads to faster slewing of the coil current, and thus to an increased trend towards dc-dc converters operating in discontinuous conduction mode.